It has long been known in packaging technology to employ, as packaging material, thermoplastic material or materials provided with thermoplastic coatings. The advantage with these materials (which, for example, may consist of polyolefins such as polythene, polypropylene but also other types of thermoplastic materials), is that they can easily be sealed or fused to one another in a tight and durable seal by surface fusion of the materials, which is achieved in that the materials are heated to their melting temperature, whereafter they are compressed at the same time as heat is dissipated and the thus formed sealing seams or joints are allowed to stabilize.
For example, it is common in the production of single use disposable packages for drinks such as milk, fruit juice etc., to employ a packaging material consisting of a base or core layer of paper or board, the base layer displaying on each side coatings of thermoplastic material, preferably polyethylene. The production of these packaging containers normally takes place with the aid of previously produced blanks or with the aid of a web reeled off from a magazine or jumbo reel. In many cases, the packaging container is produced by fold forming of the material and by heat sealing of surface portions laid together in the fold forming operation in order, by such means, to retain the shape of the package created by the folding operation. After filling of the packaging container, the container can be liquid-tight sealed by means of heat sealing.
Normally, this heat sealing is carried into effect by means of thermal energy which is supplied through heated sealing jaws, or with the aid of heat generated in the packaging material, for example by the induction of eddy currents along the desired sealing area in a metal foil layer specifically provided in the packaging laminate. In this latter case, the heat is generated in the metal foil layer and thereafter led from the metal foil layer to adjacent thermoplastic layers, which are heated to melting temperature for the purposes of surface fusion with adjacent thermoplastic layers.
One form of generation of sealing heat on the sealing of packages is to employ ultrasonic sound. In ultrasonic welding, the material is affected by a mechanically oscillating tool which is generally designated a horn, the tool operating against a back-up surface which is normally entitled an anvil. The materials intended for sealing are accommodated between the horn and the anvil and the heat requisite for the sealing or welding occurs as a result of friction in those parts of the thermoplastic material which are exposed to the friction processing. The processing frequency is not immediately critical but must, nevertheless, be so high that the heating time does not become excessively long and, in practice, the processing frequency employed in the friction welding must be above the audibility range, since otherwise the discomfort caused by noise pollution would be considerable. Thus, it is possible to carry out friction welding at a frequency within the audible range, but for practical reasons this is not applied and, as a result, this particular technology has been entitled ultrasonic welding.
Those apparatuses which are available today for heat sealing or welding thermoplastic materials with the aid of ultrasonic vibration require, in addition to an oscillating body transferring the vibrations, the so-called horn, also an oscillation generator connected to the horn and often an interjacent adaptor for transferring the vibrations from the vibration generator to the horn. The commonest method of generating the oscillations is to employ piezoelectric crystals which are disposed in a stack or battery. These crystals are energized with current from an electric high frequency generator at suitable supply frequency, i.e., approx. 20 kHz. The vibrations generated in the crystal stack are transferred via a transferring device, a so-called booster, to the oscillator or the horn which, when it is brought into contact with the material which is to be fused, generates friction losses in the contact surface and internally in the material, which is heated to the melting point. The inconvenience inherent in the prior art apparatuses is that they are bulky and require space, since each of the interconnected parts must be of a certain length. The design and length of the horn itself is determined by the frequency at which the horn is to operate and the height of the crystal stack is determined by the amplitude or vibration effect with which it is intended to operate. In addition to the fact that prior art ultrasonic sealing or welding apparatuses take up considerable space, they are also of large mass, which is a disadvantage given that they must be movable to be brought into engagement with the sealing object. A further drawback is that the prior art ultrasonic sealing apparatuses are expensive.